Gravitational lens makes dark energy less mysterious

Researchers use gravitational lensing to estimate the structure of the …

The concept of a cosmological constant—a tendency for empty space-time to either expand or contract—picked up a bad rep in part because Einstein apparently called it his greatest blunder. Einstein's problem, however, was that he included it in order to create a static Universe. When it became clear that the Universe was expanding, he quickly dropped the term from the equations he used to describe space-time. Over the past decade or so, the cosmological constant has seen a bit of a renaissance, as cosmologists have found that the Universe isn't just expanding, but that its expansion is accelerating.

We're still stumped as to what constitutes dark energy or how it produces a repulsive cosmological constant. But we are beginning to figure out just how much of the stuff is out there. A new paper, being released by Science today, provides a new and independent measure of the value of the cosmological constant. And, because it overlaps with those from different sources, it reduces the error on that estimate by 30 percent.

It's pretty clear that the current uncertainty about dark energy makes a lot of people uneasy, but the international team behind the new paper—there are almost as many countries as authors involved—open the paper with a rundown of the evidence for dark energy. Although the accelerated expansion was first detected in the spectra of supernovae, other evidence has since come from the WMAP probe's study of the cosmic background radiation, the abundance of galaxy clusters, a phenomenon called baryon acoustic oscillations, and a gravitational lensing phenomenon called cosmic shear.

All of these suggest that about 72 percent of the Universe is dark energy, which contributes an outward pressure on space-time, producing a negative cosmological constant. But all of the estimates have significant errors, and it may be more difficult to figure out what something is if we don't understand how much of it is around.

The new paper provides an independent measure of the Universe's mass-energy density, using gravitational lensing by a galaxy cluster. Gravitational lensing occurs when a massive foreground object distorts the structure of space-time itself. Light from objects in the background gets shifted as it passes through the lens, creating multiple images of some structures. The lensing depends on the amount of matter present in the object doing the lensing, and on the geometry of the Universe between the lens, the object, and the observers. The authors use this geometric information to generate an estimate of the cosmological constant.

The galaxy cluster in question is called Abell 1689, and images of it (and lensed background objects) were captured by the Hubble's Advanced Camera for Surveys. All told, Abell 1689 lenses 34 distinct background objects into a total of 114 different images. Spectroscopy, performed by the Keck and Very Large Telescopes, provides distance information for 24 of these.

The authors used a model of the structure of Abell 1689, and matched it to the images using a model that included free parameters for various constants that reflect (among other things) the structure of the Universe. The best fits for these constants were estimated using a random sampling method called Markov Chain Monte Carlo sampling. The authors also calculated the errors contributed by various factors, including the optical resolution of the Hubble (minimal), the presence of other structures along the line of sight (significant), and unknown structural differences within the Abell 1689 cluster (also significant).

Assuming the ΛCDM model—a flat Universe (based on WMAP data) and the presence of cold, dark matter—the authors were able to calculate that the matter density is about 0.3, and the cosmological constant about -1. The errors were somewhat large, but they arose from sources that were unrelated to the errors present in other estimates, such as those derived from the WMAP data and supernova surveys. As a result, and because the values overlapped, the estimates could be combined, which dropped the overall errors by about 30 percent.

The end result: at a 99 percent confidence, the matter density is between 0.23 and 0.33, while the cosmological constant is between −1.12 and −0.82.

It's not clear yet whether having a more refined measure of its presence will bring us any closer to identifying the properties of dark energy, but the fact that it shows up as a consistent value in a number of independent measures suggests that it may not be as mysterious as its name implies.